Heat treating food products for increased shelf-life is a well-known and often employed method in the art. The food products may, for example, be various dairy produce such as milk, cream or yoghurt. The heat treatment may take place in a plurality of ways, directly or indirectly. Indirect methods are, for example, heating using different types of heat exchangers. Of the direct methods there are two major groups, injection or infusion with steam. By employing a direct method, there will be obtained a very rapid heating, which is highly desirable today when, in order to improve the flavour properties, for example, in milk, the intention is many times to heat the product to elevated temperatures for a short period of time.
The present invention relates to an apparatus where use is made of infusion heating. Infusion entails heating a finely divided liquid in a steam chamber. The principle of heating a liquid, for example a liquid food, by injecting the liquid into a chamber filled with steam has been known from the early 19th century.
The first infusion heating plants were based on the same principle as condensers, with which excess steam from various chemical processes was condensed. The prior art plants thus included an infuser which consisted of a pressure vessel with an inlet for the product in the upper region of the pressure vessel. A distributor chamber discharged in the inlet, the distribution chamber finely dividing the incoming product into small, fine liquid droplets. In the bottom of the pressure vessel there is an outlet for the heated product. The pressure vessel was further provided with an inlet for steam which, in these early infusers, was located in the lower region of the pressure vessel. The steam which, at a low level in the pressure vessel, is injected in towards the finely divided liquid droplets, causes the droplets to change direction. This was a disadvantage when these infusers were, for example, employed for milk. Many of the droplets adhered to the hot inner walls in the infuser, where fouling by the product was the immediate consequence. Fouling entails major hygienic problems and is very difficult to wash off.
One way of obviating the problem inherent in fouling of product on the inner walls of the infuser is to place the steam inlet in the upper region of the infuser. The steam is supplied from above over the distribution chamber of the product so that the droplets are not disturbed in their fall down through the steam chamber. But the admission of steam concurrently with the admission of product also has its drawbacks. This type of steam admission can give rise to fouling of the product around or in the distribution holes or distribution gaps which the distribution chamber of the product inlet displays. This disturbs the heating of the product, the temperature of the product falls, in which event the steam pressure must be increased, and higher temperature differences will be obtained between product and steam. By designing the distribution holes so that they have a very thin edge out towards the steam chamber, this problem can be solved. By thus having extremely thin fouling which is easily broken off, these foulings will disappear before they become disruptive.
But the concurrent admission of steam also involves problems with the incondensable gases which are always found in the product and which are given off in connection with its being heated. The steam also contains a part of theses gases. The gases may consist of oxygen, nitrogen, carbon dioxide and others. These gases must be continuously removed in order to be able to maintain the steam temperature at the same pressure. As a result of a concurrent steam admission, the incondensable gases mix more easily with the steam and it becomes more difficult to remove them without unnecessarily releasing steam from the process as well.
Swedish Patent Specification SE 513 414 describes an infuser which has the steam inlet placed low down in the infuser vessel. As a result, space is created for the incondensable gases to accumulate in the upper region of the vessel, from whence they may be removed from the process in a simple manner. The steam inlet consists of a concentric aperture in the casing surface of the vessel. But this positioning of the steam inlet entails that hot steam enters at high speed into the vessel relatively close to the accumulation of product which is normally formed in the lower region of the vessel before the product departs from the vessel. Steam can then entrain with it product splashes on its way upwards in the vessel and undesirable fouling will be the result.